This invention relates to a cross polarization compensation system by which, in a radio communication system with dual-polarization, cross polarization interference due to the nonisotropic property of the phase characteristic of a radio channel is removed to obtain two orthogonally polarized waves.
In a radio communication system, there is a frequency re-use system which is able to double the channel capacity by using orthogonally polarized waves of the same frequency (for example, a vertical linearly polarized wave and a horizontal linearly polarized wave, or a right hand circularly polarized wave (RHCP) and a left hand circularly polarized wave (LHCP)). In the practical use of this system, it is necessary that cross polarization interference caused in transmitting and receiving antennas and in a propagation path should be held below a permitted value. The cross polarization which is yielded by antennas can be reduced sufficiently small by techniques developed in recent years. On the other hand, the cross polarization on the propagation path is mainly due to raindrops. When a rain becomes heavier, the raindrop becomes flatter, and between the electric field component in the direction of the longer diameter of the raindrop and that in the direction of the shorter diameter, there are produced a differential attenuation (hereinafter referred to simply as DA) and a differential phase shift (hereinafter referred to simply as DPS), by which cross polarization is yielded to cause interference. As the cross polarization due to the rainfall continuously changes, adaptive compensation is required which minimizes the polarization interference following the continuous change of the rainfall. For compensating for the cross polarization interference there is a system which compensates for the cross polarization due to DPS in a first compensating portion and the cross polarization due to DA in a second compensating portion. With one such system, the cross polarization resulting from DPS is compensated for by rotatable differential 90.degree. and 180.degree. phase shifters, such as a circular waveguide with a dielectric plate inserted therein, and the cross polarization resulting from DA is cancelled by a cross coupling circuit provided at a subsequent stage (Japan Pat. Disc. No. 115717/76). Moreover, there is a second system which employs a DPS compensating portion identical with the abovesaid one but differs in principle from the above first method (See 1977 IEEE International Symposium Digest AP-S, PP. 173-176, "Broadband Adaptively-Controlled Polarization Network"). The above two systems have defects such that the angle of phase shifters cannot be uniquely determined, and that the response speed of the phase shifters is not so high.